The Activated Adsorption of Nitrogen on a Finely Divided Tungsten Powder

Abstract

Two recent papers from this Laboratory have reported the results of hydrogen adsorption measurements on two finely divided tungsten powders.8 The results of the study were unique from both an experimental and theoretical viewpoint. The present investigation was undertaken to determine the detailed nature of nitrogen adsorption on tungsten powder under conditions similar to those employed in the study of hydrogen adsorption. As tungsten is a fairly active synthetic ammonia catalyst, it is of interest to know both the nitrogen and hydrogen adsorption characteristics for this metal. ExperimentalTungsten Powder.-Tungsten powder obtained from Dr. W. D. Coolidge of the General Electric Co. and marked Y-9799 was used in the investigation This was identical with one of the powders used by Frankenburg.3 Low temperature nitrogen adsorption measurements, when plotted by the BET method,4 give a surface area of 2.81 m.2/g. During the reduction and outgassing of the powder, some sintering occurs such that after this pretreatment the surface corresponds to 2.00 m.2/g., a value which was reproduced with several powder samples after their pretreatment.Gases.-The purification of the hydrogen used in the reduction of the tungsten powder has been described in the earlier paper.3 The nitrogen for adsorption purposes was prepared by the thermal decomposition of sodium azide. The sodium azide was obtained from A. D. MacKay Co., New York, N. Y., and contained 1% sodium oxide as impurity. The nitrogen evolved at 350 0 was passed through a cold trap at -195°and was collected in an evacuated bulb. The gas contained none of the rare gases as impurity and was better than 99.9% pure.

Statistical Mechanical Study on the Freundlich Isotherm Equation

“…As previously stated, for given values of N A and N S , the values of N AA are different for different configurations. Now, the N AA is assumed to be averaged over different configurations by the Bragg-Williams approximation, namely (21) [3] where N AA is the mean of N AA , and z is the coordination number characteristic of the adsorption system. Substituting Eqs.…”

“…Substituting Eqs. [2] and [3] into Eq. [1], we obtain the mean of the lateral interaction energy between the N A adsorbed molecules on N S adsorption sites (U )…”

“…Furthermore, Hayward and Trapnell found that the product of the inverse slope of a log-log isotherm and temperature (nT) decreases linearly with increasing temperature for the chemisorption of hydrogen on tungsten powder (12), studied by Frankenberg (2), and that of nitrogen on tungsten powder (12), studied by Davis (3). (The adsorption data of these chemisorptions are given in Table 1 and Table 2, respectively.…”

“…The originally empirical Freundlich isotherm equation (1) has been found to be obeyed by many experimental data of adsorption, some of which are of significance (2)(3)(4)(5). Regardless of its success in the experimental aspect, the Freundlich isotherm equation is far from being theoretically clear (6 -10).…”

Adsorption

“…Further work on the sorption of hydrogen and nitrogen on iron (49) corroborated the known facts that the former is chemisorbed as ions or atoms and the latter forms nitrides with the surface atoms. The nitrogen-tungsten system was similar to the hydrogen-tungsten and energies of activation were demonstrated to fall off as coverage of surface increased (37).…”